This invention relates to a fluid displacement apparatus and, more particularly, to a scroll type compressor having improved spiral elements on its scroll members.
Scroll type fluid displacement apparatus are well-known in the prior art. For example, U.S. Pat. No. 801,182 to Cruex discloses a scroll type apparatus including two scroll members each having a circular end plate and a spiroidal or involute spiral element. These scroll members are maintained at an angular and radial offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at lest one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, therefore, the fluid pockets change in volume. Since the volume of the fluid pockets increases or decreases, depending on the direction of the orbital motion, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
Referring to FIGS. 1a-1l and FIG. 2, the general operation of a typical scroll type compressor will be described. FIGS. 1a-1l schematically illustrate the relative movement of interfitting spiral elements to compress the fluid. FIG. 2 diagrammatically illustrates the compression cycle in each of the fluid pockets.
Two spiral elements 1 and 2 are angularly and radially offset and interfit with one another. FIG. 1a shows that the outer terminal end of each spiral element is in contact with the other spiral element, i.e., suction through suction ports 3 just has been completed, and a symmetrical pair of fluid pockets A1 and A2 just have been formed.
Each of FIGS. 1b-1l shows the state of the scroll members at a drive shaft crank angle which is advanced 90.degree. from the state shown in the preceding figure. Throughout the states shown in FIGS. 1a-1f, the pair of fluid pockets A1 and A2 shift angularly and radially towards the center of the interfitting spiral elements with the volume of each fluid pocket A1 and A2 being gradually reduced. Fluid pockets A1 and A2 are connected to one another in passing from the state shown in FIG. 1f to the state shown in FIG. 1g and, as shown in FIG. 1i, both pockets A1 and A2 merge at the center portion A and are completely connected to one another to form a single pocket. The volume of the connected single pocket is further reduced by a drive shaft revolution of 90.degree. as shown in FIGS. 1i-1k. During the course of relative orbital movement, outer spaces which are open in the state shown in FIG. 1b change as shown in FIGS. 1c and 1d to form new sealed off fluid pockets in which fluid is newly enclosed (FIG. 1e shows this state).
Referring to FIG. 2, the compression cycle of fluid in one fluid pocket will be described. FIG. 2 shows the relationship of fluid pressure in the fluid pocket to crank angle, and shows that one compression cycle is almost completed at a crank angle of 5.pi., in this case.
The compression cycle begins (FIG. 1a) when the fluid pockets are sealed, i.e., the outer end of each spiral element is in contact with the opposite spiral element, the suction phase having finished. This state of fluid pressure in a fluid pocket is shown at point H in FIG. 2. The volume of the fluid pocket is reduced and fluid is compressed by the revolution of the orbiting scroll until the crank angle reaches approximately 3.pi., which state is shown by point L in FIG. 2. Immediately after passing this state and, hence, passing point L, the pair of fluid pockets are connected to one another and simultaneously are connected to the space filled with high pressure fluid, which is left undischarged at the center of both spiral elements. At this time, if the compressor is not provided with a discharge valve in discharge port 4, the fluid pressure in the connected fluid pockets suddenly rises to equal the pressure in the discharge chamber. If, however, the compressor is provided with a discharge valve, such as a reed valve which will open at a predetermined discharge pressure, the fluid pressure in the connected fluid pockets rises only slightly due to mixing of the high pressure fluid and the fluid in the connected fluid pockets. This state is shown at point M in FIG. 2. The fluid in the high pressure space is further compressed by orbital motion of the orbiting scroll until it reaches the discharge pressure. This state is shown at point N in FIG. 2. When the fluid in the high pressure space reaches the discharge pressure, the fluid is discharged to the discharge chamber through the discharge port by the automatic operation of the reed valve. Therefore, the fluid in the high pressure space is maintained at the discharge pressure until a crank angle of approximately 5.pi. (point A in FIG. 2) is reached. Accordingly, one cycle of the compressor is completed at a crank angle of 5.pi., but the next cycle begins at the mid-point of compression of the fluid cycle as shown by the dashed lines in FIG. 2. Therefore, fluid compression proceeds continuously by the operation of these cycles.
In this type of scroll compressor, the wall thickness of each spiral element from its outer terminal end to its inner end is uniform. Generally, the wall thickness of each spiral element will be designed as a predetermined minimum thickness required for spiral strength, since the largest possible fluid volume must be accommodated within the predetermined diameter of the compressor housing. The various factors affecting spiral element strength must be considered in scroll member design. During the operation of the compressor, for example, the spiral elements, which define the sealed off fluid pockets, are subjected to cyclical changes of fluid pressure, which may cause fatigue rupture of the spiral elements. The inner end portion of the spiral element--the terminal portion located at the high pressure space--is especially vulnerable to fatigue because it can flex more easily than a central portion of the spiral. The central portion itself is vulnerable in the case of a lengthened spiral element (formed longer to obtain a large compressor displacement) because of reduced spiral rigidity. The spiral element can be strengthened by uniformly increasing the wall thickness, but if the displacement of the compressor is to be kept the same, the dimensions of the casing must be increased, resulting in a larger and heavier compressor.
Generally, an end milling tool is used for forming the spiral element on the scroll member. Such a milling tool must have a certain minimum diameter in order to be rigid enough so that fine finishing of the spiral element can be carried out. A sufficiently rigid tool, however, has a diameter which is too large to permit the milling of the inner side wall of the spiral (at the inner end thereof) in a shape which properly follows the desired involute curve and properly intersects the involute generating circle. An undesirable arc-shaped configuration results on the inner side wall of the inner end portion of the spiral element, having a radius which matches that of the milling tool.
During operation of a compressor which includes the above-configured spiral element, the line contacts defined between the involute curved surfaces of the spiral elements are dissolved when the line contacts reach the inner end portion of the spiral elements which have the undesirable arcuate configuration. At this time, the central high pressure pocket within which high pressure fluid remains is connected to the adjacent pair of fluid pockets. Therefore, the high pressure fluid within the high pressure pocket is partially re-expanded, resulting in a loss of power and a reduction of efficiency.